1. Field of the Invention
This invention relates to the treatment of a sorbent, such as activated carbon, and/or fly ash to remove mercury that contaminates the sorbent and fly ash as part of post-combustion treatments of exhaust gases from a coal-fired power plant.
2. Description of the Related Art
In 1990, the United States Environmental Protection Agency (“EPA”) put into place the Clean Air Act Amendments which were designed to reduce the emissions of “greenhouse gases”. Among the emissions covered are the nitrogen compounds NO and NO2, referred to generically as NOx. NOx is generated through the combustion of coal and its generation is directly affected by combustion temperature, residency time, and available oxygen. Several technologies have been developed to meet the mandated NOx reduction limits. One category includes technologies that are employed after combustion has taken place. These technologies include selective non-catalytic reduction, selective catalytic reduction, and amine enhanced fuel lean gas reburn. These technologies involve adding ammonia to the exhaust gases, and a significant amount of the ammonia finds its way onto the fly ash, typically by combining with available sulfur and other compounds that attach to the ash particles.
Fly ash is a marketable product if it is not contaminated. The ash may be used, for example, in concrete products as a replacement for a portion of the cement. However, fly ash that has been treated to reduce NOx and which is contaminated either by unburned carbon or ammonia compounds is not marketable. Systems have been developed which may be used to reduce the amount of ammonia compounds affixed to fly ash. For example, PCT International Patent Publication No. WO 01/12268 describes a process for the reduction of ammonia residues from the recovered fly ash of a coal fired power plant.
In addition, the emission of mercury compounds from all sources, including coal-fired power plants, has drawn national and international attention due to the fact that certain forms of mercury have deleterious effects on wildlife and are toxic to humans. Mercury is a naturally occurring element in the environment; however, human activities over the centuries have released large quantities of this element from its sequestered forms (mercury-containing ores, soils, rocks, including all forms of coal). Currently, scientists believe that most of the mercury entering the environment results from air emissions. Large scale releases from certain mining activities (e.g. gold mining), coal burning, medical and municipal waste incineration appear to be the largest anthropogenic sources. However, natural degassing from the oceans, soils, and rocks is thought to be the largest overall source of mercury to the atmosphere.
Mercury emitted from the above sources is transported and transformed by atmospheric processes that are only partially understood. However, it is known that when the oxidized form of mercury (currently believed to constitute a very small percentage of all mercury in the atmosphere) deposits to certain aquatic systems, such as wetlands, salt marshes, and certain lakes, this form of mercury undergoes chemical transformation by certain microbes. These microbes convert the inorganic form to methylmercury, a very potent neurotoxin. While this form is typically present in very low concentrations in the environment, it can be bioaccumulated via the food chain. The mercury levels in the top members of the food chain are often present at concentrations thousands of even millions of times greater than what can be found in natural waters. These higher concentrations are found in fish or mammals that occupy the top of ecosystem food chains. Persons who eat large quantities of these fish are thought to be at risk from developing mild to severe forms of mercury poisoning. Women who eat large quantities of mercury-contaminated fish or seafood and are pregnant, run the risk of giving birth to a child who may experience learning disabilities.
The United States Environmental Protection Agency (EPA) is focusing on mercury, because mercury has been identified as a toxic of great concern among all the air toxics emitted from power plants. To reduce the risk mercury poses to people's health, the EPA is announcing that it will regulate emissions of mercury and other air toxics from coal- and oil-fired electric utility steam generating units (power plants). The data indicates that coal-fired power plants are the largest source of human-caused mercury emissions in the United States. It has been reported that the EPA is likely to propose mercury regulations by Dec. 15, 2003 and issue final regulations by Dec. 15, 2004.
Physical forms of mercury in ambient air can be divided into two categories: vapor phase, which is dominant in the atmosphere, and particulate phase (associated with aerosols), which only comprises a few percent of total airborne mercury emissions. Chemical forms determine the transport of mercury between different environmental media (air, water and soil). The mercuric compounds can be classified into elemental and divalent forms. The elemental form of mercury (Hg0) is the dominant form (>98%) of vapor-phase mercury in the atmosphere, and following dissolution in cloud water or rainwater, is readily converted to more soluble mercury species. Elemental mercury possesses relatively high vapor pressure and low solubility. The former property leads to considerable mercury evaporation into the ambient air, while the latter makes it difficult for the existing air pollution control devices to remove mercury from the emission sources. Divalent mercury forms include inorganic (Hg2+, HgO, HgCl2) and organic oxidized forms (CH3Hg, CH3HgCl, CH3HgCH3). Divalent forms possess higher solubility and readily combine with a variety of reactants, such as sulfite, chloride and hydroxide ions, in the aqueous phase. The boiling points of elemental mercury and some mercury compounds are as follows: Hg, 356.58° C.; HgCl2, 303° C.; and HgS, 580° C.
Many existing air pollution control technologies and several innovative methods have been evaluated for the control of vapor-phase mercury emissions from combustion processes. Sodium sulfide (Na2S) has been used for vapor-phase mercury control in municipal solid waste combustors in Canada, Sweden, Germany and British Columbia. Sodium sulfide injection is usually combined with dry sorbent injection and fabric filters for acid gas and particulate matter control. It has been reported that mercuric sulfide (HgS) is generated as a fine particulate in the process, which may prove difficult to capture in less efficient electrostatic precipitators. Other potential problems for this process include corrosion, hydrogen sulfide formation and chemical storage and handling. These problems, compounded by the lack of test data on full-scale coal-fired power plants, cloud the utility of sodium sulfide injection for the control of mercury emissions. (See Sengupta, “Environmental Separation of Heavy Metals: Engineered Processes”, CRC Press, 2001.)
Wet scrubbers have been routinely used to remove hydrochloric acid and sulfur dioxide from the flue gases of industrial factories, coal-fired power plants and municipal waste combustors. Considerable interest in the use of wet scrubbers systems to simultaneously remove sulfur dioxide and mercury has recently been expressed. The removal of vapor-phase mercury in the wet scrubber system would also occur by absorption in the scrubbing slurry, whereby the mechanism of mercury removal depends on the solubility of mercury in the scrubbing slurry, contact time and solution chemistry. Elemental mercury is essentially insoluble in the wet scrubbing slurry, while some of the oxidized species, such as mercuric chloride, are highly soluble. Therefore, oxidized mercury can be easily absorbed with sufficient gas-liquid contact, while the removal of elemental mercury would remain limited. Chang and Owens reported that the treatment of a coal-fired power plant flue gas using only a wet scrubber allowed 70–75% of elemental mercury to be discharged into the atmosphere (see Chang and Owens, EPRI J., 16, 2, 183–189,1994), while other studies reported 30–70% removal of elemental mercury by wet scrubbers. (see, Sengupta, above).
Presently, the most effective and widely used technology for capturing mercury from flue gas emissions is to inject activated carbon into the gas stream. Injected activated carbon binds the vapor-phase mercury through physical adsorption and chemisorption and is collected in downstream particulate collection devices, such as fabric filters (baghouses) or electrostatic precipitators. Results from several tests indicated that effectiveness of activated carbon injection in removing mercury vapor depends on the type and composition of burned materials, flue gas composition and temperature, mercury speciation, activated carbon properties and injection rate and operating conditions. Because activated carbon can be collected effectively in the existing particulate control devices, direct activated carbon injection has several potential advantages over wet scrubbing processes.
One method of doing this is to inject powdered activated carbon into the exhaust gas upstream of a primary particulate collector (e.g., an electrostatic precipitator or baghouse). However, when using this method, the carbon/mercury mixture is collected along with fly ash. The collected fly ash has a higher carbon content (from the activated carbon) and has increased mercury levels due to the mercury adsorbed on the activated carbon. As a result, the collected fly ash becomes unusable in concrete without beneficiation to remove the additional carbon content. Thus, the value of the resulting fly ash declines because of more limited uses and the need for expensive beneficiation techniques such as froth flotation, electrostatic separation, or reburning the fly ash. Because the primary use of fly ash includes cementitious material for concrete and concrete products, feed stock for Portland cement manufacture, liquid waste stabilization, and lightweight aggregate production, it is essential to maintain the high quality of fly ash for use in concrete. Also, mercury adsorbed by the activated carbon may increase the potential for release of mercury into the environment during reuse or landfilling of the fly ash.
Another method for capturing and removing mercury from exhaust gases involves injecting powdered activated carbon into the exhaust gas downstream of the primary particulate collector and ahead of a secondary particulate collector (e.g., an electrostatic precipitator or baghouse). The resulting carbon/mercury mixture is then collected in the secondary particulate collector for disposal. Therefore, the quality of fly ash collected in the primary particulate collector is warranted for reuse in concrete. However, the issue still remains as to what to do with the carbon/mercury mixture collected in the secondary particulate collector. As stated above, the mercury adsorbed by the activated carbon may increase the potential for release of mercury into the environment during landfilling or other disposal of the mixture. Furthermore, the expense associated with current activated carbon injection technology can be quite high due to the disposal costs associated with mercury contaminated carbon.
Therefore, there is a need for an improved method and apparatus that can remove adsorbed mercury from a sorbent, such as activated carbon, that is collected separately or collected with fly ash in an exhaust gas treatment process for a coal-fired power plant.